The present invention relates to a steering mechanism for wheeled vehicles, and more particularly to an improvement of a steering mechanism of the rack-and-pinion type.
In general, a conventional rack-and-pinion type steering mechanism includes a gear housing integrally formed at one side thereof with a radial cylinder portion, a pinion shaft rotatably mounted within the gear housing and being operatively connected to a steering wheel, a rack member axially slidably carried on the gear housing across the pinion shaft and being in mesh with the pinion shaft at the toothed portion thereof to be moved in response to rotation of the pinion shaft, a guide member slidably disposed within the radial cylinder portion of the gear housing to be moved toward and away from the rack member and having a thrust surface for slidable engagement with the rear surface of the rack member, a closure member adjustably threaded into the radial cylinder portion of the gear housing, and a compression spring interposed between the guide member and the closure member to bias the guide member toward the rack member.
In the steering mechanism described above, it is desirable that the operator's effort applied to the steering wheel is reduced during low speed travel of the vehicle and is weighted during high speed travel of the vehicle. For such control of the operator's effort in his steering operation, a steering mechanism has been proposed in an Early Publication of the Japanese Utility Model Application No. 54-138239, wherein the space between the guide member and the closure member is formed as a pressure chamber arranged to be supplied with fluid under pressure proportional to rotational speed of a prime mover of the vehicle. With such an arrangement of the pressure chamber in the steering mechanism, the sliding resistance of the rack member can be adjusted in accordance with the rotational speed of the prime mover so that the operator's effort in his steering operation is reduced during low speed travel of the vehicle and is weighted during high speed travel of the vehicle. Furthermore, the hydraulic pressure acting on the rack member serves to increase the support rigidity of the rack member at its meshed portion with the pinion shaft and to decrease a backlash between the pinion shaft and the rack member. However, the guide member does not act to restrict rotation of the rack member in a circumferential direction. For this reason, the rotation of the rack member causes a circumferential interspace between the rack member and the pinion shaft, resulting in decrease of the support rigidity of the rack member.